Continuous sputtering system

ABSTRACT

In a system for depositing thin films of material on a workpiece by a technique known as radiofrequency sputtering, an electrical transformation bridge comprised of inductive and capacitive components is electrically interfaced between an unbalanced output of a radiofrequency generator and the excitation electrodes of a sputtering chamber for converting the generator output signal into opposing phase radiofrequency voltage components employed in preferred continuous sputtering systems and for precisely matching the impedance output of the generator with the impedance of the excitation electrodes while the latter are under the influence of a plasma of charged deposition particles.

United States Patent 3,525,680 8/1970 Davidse et al. 3,537,973 11/1970 l-lerte et a1.

ABSTRACT: In a system for depositing thin films of material on a workpiece by a technique known as radiofrequency sputtering, an electrical transformation bridge comprised of inductive and capacitive components is electrically interfaced between an unbalanced output of a radiofrequency generator and the excitation electrodes of a sputtering chamber for converting the generator output signal into opposing phase radiofrequency voltage components employed in preferred continuous sputtering systems and for precisely matching the impedance output of the generator with the impedance of the excitation electrodes while the latter are under the influence of a plasma of charged deposition particles.

Tunic/IRE) I OUTPUT MATCHING I NETWORK "83 a4 L? T Z1 7 u 16 2s 38 W/- AI'RANSFORMATION BRIDGE Q- 'Z- DUAL 3| ELECTRODE Ru H 44 3 SPUTTERING T-K: CHAMBER I 34-- I 32 3? I 42 CONTINUOUS SPUTTERING SYSTEM The present invention relates to systems for depositing thin films on a workpiece by employing an alternating electric field proximate a source of deposition material to dislodge charged particles or ions therefrom for developing a plasma of charged deposition particles which propagate toward and are deposited on the workpiece. Particularly, the present invention pertains to method and apparatus for transforming energy between the output of an alternating current generator and the electric field required for developing the plasma of deposition ions.

A film deposition technique having the foregoing characteristics is commonly referred to in the art as a sputtering system. in operation, a source of the material to be deposited, referred to as the target, is disposed continuously with one or more electrodes adapted to receive an alternating voltage signal, usually in the radiofrequency range, for generating the excitation electric field. The source (or target) and the electrodes are disposed together with a workpiece within a chamber having a subatmospheric gas pressure, whereby an ion plasma generated by the high-frequency electric field dislodges charged or ion particles from the source which are dispersed toward the workpiece for deposition of a cohesive layer of particles thereon. Deposition processes of this type have proved exceedingly advantageous in the manufacture of miniaturized semiconductor circuits, such as thin film circuits, where successive steps of deposition and masking of a workpiece are employed to form numerous electrical circuit components on a small substrate.

Several apparatus configurations have been employed for deposition of thin films by sputtering. For example, in one known system, the RF electric field is generated between a conductive work table which is electrically grounded and a single discoidal nongrounded electrode spaced from the worktable and to which the target or source material is attached. The ground terminal from the RF generator is connected to the worktable while the nongrounded output table is connected to the discoidal electrode. A system having this arrangement of components is sometimes referred to as a grounded diode sputtering device and is relatively simple to construct, particularly as to the means for interconnecting the output of the generator with the electrode and worktable of the sputtering chamber. For example, such interconnection may be effected by a low cost coaxial cable connection between the sputtering chamber and an unbalanced output of an RF generator. However, in the operation of such units, it has been discovered that the deposition of the source ions on the workpiece proceeds at an undesirably slow rate due to polarity considerations and the formation of a charge layer barrier adjacent the material source during each half cycle of the applied alternating voltage waveform. Deposition occurs during a positive half cycle defined by the discoidal electrode being positive relative to the worktable whereas no deposition occurs during a negative half cycle in which these respective polarities are reversed. Furthermore, during the latter half of the positive cycle, a layer of charged particles develops adjacent the source material and forms a barrier preventing further deposition during the positive half cycle. However, this layer of charge is dissipated in the negative half cycle and the source material and surrounding ion plasma are restored to a charged condition permitting ion deposition during the succeeding positive half cycle. In this respect, the deposition of the workpiece proceeds in a relatively slow pulsating rate analogous to the conduction through a half wave rectifier or diode from which this system derives its name.

In order to improve the deposition characteristics, i.e. uniformity and rate, there has evolved in the art a sputtering system employing a pair of excitation electrodes rather than the single excitation electrode of the grounded diode apparatus in which the pair of electrodes are energized relative to the work table with radiofrequency voltage signals of opposing phase, i.e. 180 out of phase. By virtue of this construction, deposition proceeds at a continuous rate, in which the source ions are carried to the worktable and workpiece in response to the adjacent positive half cycles alternately provided by the pair of excitation electrodes relative to a grounded worktable. To insure uniformity of deposition, the electrodes are shaped and arranged in a symmetrical configuration, in this instance consisting of a central disc electrode and an annular ring electrode disposed in radially spaced and concentric relation. By reason of the improved deposition speed and uniformity of the resulting deposited films, this dual electrode sputtering system is preferentially used in the art. However, in the construction thereof, it has heretofore been impossible to provide an efficient yet economical means for generating the required opposing phase RF driving signals and at the same time efficiently conveying the electrical energy to the pair of excitation electrodes mounted in the sputtering chamber. For example, it has been known to derive the opposing phase RF signals from an RF generator designed with a balanced output network, which consists of a pair of output power tubes coupled through a transformer to an output, wherein the secondary or output winding of the transformer has a grounded center tap and the opposing phase waveforms are developed relative to ground at each end of the secondary winding. Such circuits are not only costly in terms of materials and construction, but moreover the transformer coupling is exceedingly lossy such that the transformation between the power input delivered to the tubes and the signal power output derived across the secondary winding of the transformer is highly inefficient. In addition to this substantial insertion loss of the transformer coupling, such output circuits require a balanced shielded line [two spaced parallel conductors for the opposed phase RF signals and a surrounding shield conductor for ground] to convey the electrical energy from the generator output to the electrodes of the sputtering chamber. Such balanced lines are inherently large, unyielding, and expensive to construct. Also, balanced transmission lines are not easily adapted for connection to electrical measuring instruments, such as a serial connection with an RF watt meter, desirable in most cases for measuring the amount of electrical energy transmitted to the sputtering chamber.

While it has been the practice to match the impedance interface between the transmission line and output of the RF generator, both for the unbalanced and balanced type generator outputs, heretofore the impedance interface between the transmission line or cable and the electrodes of the sputtering chamber has been merely approximated and in many cases entirely ignored. Because of this, a portion of the electrical energy emanating from the RF generator (which would otherwise pass to the sputtering chamber for raising the energy level of the ion plasma developed therein), is reflected back to the generator source at the connection of the transmission line to the chamber electrodes. The inefficiency of the RF energy transmission is particularly pronounced in the dual electrode sputtering system wherein much of the theoretical advantage gained by such systems in terms of deposition rate is lost by reason of this impedance mismatch at the sputtering chamber. Additionally, as the construction of the dual electrode sputtering system renders the measurement of the RF energy applied to the sputtering chamber impractical, it has been found difficult if not impossible to account for, control, and reproduce the effective energy reaching the deposition plasma. For example, one heretofore employed method for measuring the RF sputtering energy has been to monitor the amount of power applied at the input to the RF generator and to estimate the amount of losses occurring between the generator and the electrodes of the sputtering chamber. As the losses may vary, however, this method does not adequately account for the RF energy actually reaching the sputtering chamber during any given deposition sequence.

Accordingly it is an object of the present invention to provide in a system of the type characterized, method and apparatus for effectively and efficiently conveying alternating electrical energy between a generator and the excitation electrodes of the sputtering unit for'maximum utilization of power developed by the generator.

It is another object of the present invention to provide such a method and apparatus which is conveniently adaptable for monitoring the effective electrical energy actually reaching the excitation electrodes of the sputtering chamber.

It is still another object of the present invention to provide such an apparatus which lends itself to simplicity of construction for low cost manufacture and which may be easily and rapidly assembled and disassembled with respect to the generator and sputtering chamber.

Other features, objects and advantages of the present invention will become apparent from the following description to be read in conjunction with the accompanying drawings forming a part of this specification and illustrating the preferred embodiment of the invention. In the drawings:

FIG. 1 is an elevation view, partially cut away for clarity, illustrating the transformation bridge constructed in accordance with the present invention and as employed in a dual electrode sputtering deposition system;

FIG. 2 is an electrical diagram partly in schematic and partly in block form characterizing the electrical connection of the transformation bridge between an RF generator output and a dual electrode sputtering chamber such as shown in FIG. 1;

FIG. 3 is a top elevation view, partially cut away for clarity, of the transformation bridge shown in FIG. 1, however, disassembled from the sputtering chamber;

FIG. 4 is a front elevation view of the transformation bridge shown in FIG. 3 illustrating the front instrument panel; and

FIG. 5 is a bottom elevation view of the transformation bridge as shown in FIGS. 3 and 4.

With reference to FIGS. 1 and 2, the present invention comprises in general a continuous sputtering system in which an L- C [inductive-capacitive] transformation bridge 11 is connected between an unbalanced output 12 of an RF [radiofrequency1generator 13 and a dual electrode sputtering head 14 of a sputtering chamber 16 for efficiently and effectively channeling RF energy from the generator to the sputtering chamber for developing a plasma 17 of deposition particles or ions therein. Briefly, the deposition process is effected by energizing a plate 18 of source material (sometimes called target material), by an RF electric field such that ions are dislodged therefrom which enter a plasma 17 of positively and negatively charged particles. The source ions eventually propagate toward and are deposited upon a workpiece 19 to form a thin cohesive layer of the source material thereon. Here, sputtering head 14 is formed with a pair of ungrounded electrodes 21 and 22, in this instance of discoidal and annular configuration respectively, to which 180 phase opposed RF voltage waveforms are applied relative to a grounded electrically conductive worktable 23 such that preferred continuous sputtering and deposition is achieved as discussed above relative to workpiece 19.

In accordance with the present invention, the opposing phase RF signals driving excitation electrodes 21 and 22 are derived from an unbalanced output of generator 13, here provided by an unbalanced RF power amplifier 26 and an unbalanced output matching network 27, which function to issue a single phase RF voltage to an unbalanced coaxial cable 28 connected between an output 12 of the generator and input 29 of transformation bridge 11. Bridge 11, comprised of a pair of inductors 31 and 32 alternately connected end to end with a pair of variable capacitors 33 and 34, provides for the conversion of the single phase RF signal, received through cable 28, into opposing phase RF excitation voltages for driving electrodes 21 and 22 via leads 36 and 37. In addition, bridge 11 concurrently effects a precise matching of the real or dissipative impedance of the electrodes with the output of generator 13. Moreover, this impedance match is achieved during active operation of the sputtering system, wherein the real, i.e. dissipative impedance, of electrodes 21 and 22 is substantially influenced by the presence and close proximity of plasma l7 and takes on a value largely at variance with the free space electrode impedance. A number of advantages are achieved by this arrangement including principally, highly efficient transmission of RF energy between generator 13 and sputtering head 14 both in terms of low reflection losses provided by the impedance matching capability of bridge 11 and low insertion loss attributed to the use of an unbalanced circuit at the output of the generator, as compared with balanced output generator circuits which are characteristically inefficient. It is apparent that with an increased amount of energy reaching sputtering head 14 for any given system, a greater proportion of the total power consumed by the system is effective in generating ion plasma l7 and thereby enhancing the uniformity and rate of the deposition process.

Additionally, simplification in the construction of the RF generator output is realized unbalanced output circuits such as that of amplifier 26 and network 27, are inherently less complex and comprise fewer components than balanced output circuits which have heretofore been required]. An accompanying economy and convenience is provided by allowing employment of an unbalanced coaxial transmission line, such as cable 28, between the generator and sputtering unit as opposed to the heretofore used balanced transmission lines [which are more costly and of larger, unyielding construction]. Furthermore, as it is desirable to monitor the RF energy passing from the generator output to the sputtering chamber, in this instance through transformation bridge 11, this energy transfer may be conveniently measured by serial connection of a commercially available RF watt meter 38 with cable 28. Commercially available RF watt meters are universally equipped for connection with unbalanced lines, whereas the instruments must be specially adapted by the customer should a balanced line connection be required.

In the operation of transformation bridge 11, an input 29 extends the single phase unbalanced RF voltage output from generator 13 across a pair of electrically nonadjacent bridge junctions 41 and 42, wherein one of these junctions, in this instance junction 42, is extended to an earth or reference ground. By selecting the impedance values of inductors 31 and 32 and capacitors 33 and 34 in a manner more fully described below, separate RF voltage signals are issued with respect to ground from the remaining pair of electrically nonadjacent bridge junctions 43 and 44 which are extended over leads 36 and 37 to electrodes 21 and 22 of sputtering head 14. Thereat, the voltage signals appear as balanced, opposed phase RF voltage wave forms, V and V, with respect to grounded conductive portions of chamber 16, particularly grounded worktable 23. The real or dissipative impedance encountered by each of electrodes 21 and 22 in the plasma environment, appear as equivalent resistors R, and R,,, shown by dotted lines in the circuit of bridge 11, between junctions 43 and 44 and ground, respectively. In accordance with this equivalent circuit, the electrode impedance R forms a real impedance load associated with that half of the bridge comprised of inductor 31 and capacitor 34, while resistance R forms an efiective load associated with the remaining half of the bridge composed of capacitor 33 and inductor 32. By selecting the values for inductor 31 and capacitor 34 to match electrode impedance R and the values of capacitor 33 and inductor 32 to match impedance R,,, an effective transfer of RF energy is achieved between input 29 and sputtering head 14. Furthermore, as electrodes 21 and 22 are, in this instance and as a general rule, constructed to issue equal amounts of RF energy in forming plasma 17, the dissipative impedances, R and R exhibited thereby are substantially equal' Thus, inductors 3i and 32 and capacitors 33 and 34 are selected to provide not only a transfer impedance between the input to the bridge across junctions 41 and 42 and the output thereof across junction terminals 43 and 44 matching the impedance between cable 28 and the sum of equivalent resistors R, and R, [equal to the total dissipative load impedance across the bridge output], but also to provide an equal impedance in each branch of the bridge such that the voltage signals appearing on leads 36 and 37 are equal in magnitude and separated in phase by 180.

To achieve the foregoing selection of values for bridge 11 in any given system, the following formula is employed:

. w Zc ZL inZou! where,

Z impedance of coaxial cable 28 Z,,,,, total dissipative impedance of electrodes 21 and 22 during operation of the system 2 impedance of capacitors 33 and 34 at operating frequency Z, impedance of inductors 31 and 32 at operating frequency As exemplary values for a system operating at a frequency of 13.5 megacycles, with individual electrode impedance values for R and R, equal to 2.5 ohms (or Z,,,,,= ohms) and the impedance of cable 28 equal to 50 ohms (or Z,,,= 50 ohms) the value of inductors 31 and 32 is calculated to be 2.5 microhenrys and the value of capacitors 33 and 34 about 800 microfarads.

With reference to FlG.(S) 3-5, the components of transformation bridge 11 are mounted in housing 46 formed with electrically conductive bottom and top walls 47 and 48, sidewalls 51 and 52, back wall 53 and a front instrument panel wall 54. Furthermore, inductor 31 and capacitor 34 associated with output junction 43 are preferably separated from capacitor 33 and inductor 32 associated with output junction 44 into separate shielded compartments of housing 46 by means of an electrically conductive isolating wall 56 providing electrostatic and electromagnetic shielding therebetween. This precludes any Faraday coupling interaction between the pairs of associated components. Wall 47 is formed with a recess 57, in this instance of circular shape, for mounting housing 46 on chamber 16, here of cylindrical shape, with recess 57 nested on a top mated edge portion of the chamber as shown in FIG. 1. An electrically conductive cylindrical wall 58 of chamber 16 thereby is in electrical communication with bottom wall 47 of the bridge housing and serves to extend the reference ground associated with bridge 11 to conductive work table 23 via electrical connections indicated by dotted lines 61 and to an electrode shield 62 via connections indicated by dotted lines 63. The recessed portion of bottom wall 47 is formed with spaced-apart openings 66 and 67 for extending leads 36 and 37 from internally of housing 46 to connections with stud terminals 68 and 69, upwardly extending from electrodes 21 and 22, respectively. The electrically active portions of inductors 31 and 32 and capacitors 33 and 34 are insulated from the housing walls by means of standoff insulators such as insulator 71 for inductor 32. Inductors 31 and 32 and each of the interconnecting leads for bridge 11 are formed of highly conductive copper tubing and the permanent conductor joints such as at junctions 41, 43 and 44 are silver brazed to insure positive stable electrical connections. it is noted that housing 46 is at reference ground such that one of the ends of both inductor 32 and capacitor 34 are connected to housing wall 47 as shown at 42a and 42b thus forming junction 42 as shown in FIG. 2. A coaxial female connector 72 mounted with an outer conductive portion in electrical contact with housing wall 53 and an inner conductor insulated therefrom is extended into electrical engagement with junction 41 to provide the unbalanced coaxial input 29 for receiving one end of cable 28.

RF generator 3 includes an RF oscillator 76 and a preamplifier or exciter 77 for receiving the RF frequency output of oscillator 76 and driving the grid of a power amplifying tube 78 of power amplifier 26. In simplified form, amplifier 26 includes, in addition to tube 78, an RF choke 79 in series with the plate supply circuit of the tube and a DC blocking capacitor 81 through which the amplified RF signal is fed to a junction 82 with the input of network 27. In this instance matching network 27 is an L-C 11 network comprised of a pair of variable capacitors 83 and 84 and an inductor 86 connected between junction 82 and generator output 12 and serves to match the output impedance of amplifier 26 with the characteristic impedance of coaxial cable 28. While one form of an unbalanced RF generator is herein disclosed by way of example, it will be apparent to those skilled in the art that several other types of unbalanced output networks may be employed in place of those shown for amplifier 26 and network 27.

In preparing the sputtering system shown in FIG. 1 for deposition, chamber 16 is initially filled with a gas having a suitable environment for the sputtering process, such as argon, and the pressure of such a gas is thereupon reduced substantially below that of atmospheric pressure by drawing a vacuum on a port 88 communicating with the otherwise hermetically sealed interior of chamber 16. Thereafter RF generator 13 is energized so as to develop an RF electric field permeating source target 18 which in response thereto and in conjunction with the entrapped argon gas effects formation of plasma 17 of charged particles. Field control magnets 97 are frequently employed for providing a preferred direction of propagation of the deposition ions toward target 19. During alternate half cycles of the opposed phase RF voltage waveforms applied to electrodes 21 and 22, positive ions dislodged from target 18 are forced to worktable 23 and workpiece 19 first by one electrode and then by the other providing the heretofore characterized continuous sputtering operation.

in accordance with the present invention, to insure maximum transfer of electrical energy from generator 13 to dual electrode sputtering head 14, capacitors 33 and 34 are simultaneously adjusted from their central impedance values such that the power registered by RF watt meter 38 is "peaked or by obtaining a null" in case reflected power is measured by meter 38. For this purpose, manual adjustment means are provided on the instrument panel wall 54, here taking the form of knobs 91 and 92 carried by capacitor shafts 93 and 94. While the calculated values for capacitors 33 and 34 will in many cases serve to provide a highly efficient transformation of the RF energy to the dual electrode sputtering head, the adjustment thereof as above described is preferred for reaching a precision impedance match between the generator and electrodes.

lclaim:

1. In an RF sputtering system having an alternating current generator and a pair of excitation electrodes for supporting material to be sputtered the combination comprising,

an inductive-capacitive transformation bridge electrically disposed between the generator and excitation electrodes.

2. The combination defined in claim 1, wherein said transformation bridge comprises, a reference ground, a first inductor connected between said ground and a first output terminal, a first capacitor connected between said first output terminal and an input terminal and forming with said first inductor a first network of said bridge electrically associated with one of the electrodes, a second inductor connected between said input and a second output terminal, a second capacitor connected between said second output terminal and said ground and forming with said second inductor a second network electrically associated with the other electrode.

3. The combination as defined in claim 2, wherein said first and second capacitors are variable for accommodating precise impedance and phase transfonnations between said generator and electrodes under sputtering conditions.

4. The combination as defined in claim 3, wherein the impedance exhibited by each said first inductor and said second inductor and each said first capacitor and said second capacitor is substantially equal to the square root of a quantity equaling the input impedance to said bridge times the real impedance of said electrodes under deposition conditions.

5. The combination as defined in claim 2, further comprising, a housing for said bridge having separate enclosed compartments formed of electrically conductive walls and the inductor and capacitor of each said network being disposed in an individual one of said compartments for electrostatic and electromagnetic shielding therebetween.

6. An RF sputtering system for particle deposition of a material on a workpiece, the combination comprising,

waveform thereto and concurrently matching the im- 10 pedance of said generator output with the dissipative impedance of said electrodes under the influence of said plasma.

7. A system as defined in claim 6, said bridge comprising, a pair of inductors and a pair of capacitors alternately connected end to end and providing said bridge input across a first set electrically nonadjacent junctions and providing said outputs at the remaining electrically nonadjacent junctions relative to one of said first set junctions.

8. A system as defined in claim 6, further comprising, an unbalanced coaxial transmission line connecting said generator output to said bridge input.

9. A system as defined in claim 8, wherein said chamber is formed with electrically conductive wall and said bridge is mounted in an electrically conductive housing disposed in contiguous relation with said sputtering chamber and forming a reference ground therewith.

10. In an RF sputtering method of depositing particles on a workpiece by exciting a source of material with an alternating electric field issued by electrodes mounted adjacent the source material and energized by an alternating voltage output of a generator to form a plasma of deposition particles the steps comprising,

transforming the alternating voltage issued by said generator into opposing phase voltage components by applying said generator output to an inductive-capacitive bridge,

applying said opposing phase voltage components separately to separate said electrodes by electrical connections to said bridge, and

adjusting the impedance of said bridge to match the impedance of said generator output with the dissipative impedance of said electrodes while under the influence of said plasma.

11. The steps as defined in claim 10, wherein said bridge is formed by alternate inductive and capacitive branches, and said steps of adjusting comprises, simultaneously varying the capacitances of said electrodes in the environment of said plasma.

t I I i i 

2. The combination defined in claim 1, wherein said transformation bridge comprises, a reference ground, a first inductor connected between said ground and a first output terminal, a first capacitor connected between said first output terminal and an input terminal and forming with said first inductor a first network of said bridge electrically associated with one of the electrodes, a second inductor connected between said input and a second output terminal, a second capacitor connected between said second output terminal and said ground and forming with said second inductor a second network electrically associated with the other electrode.
 3. The combination as defined in claim 2, wherein said first and second capacitors are variable for accommodating precise impedance and phase transformations between said generator and electrodes under sputtering conditions.
 4. The combination as defined in claim 3, wherein the impedance exhibited by each said first inductor and said second inductor and each said first capacitor and said second capacitor is substantially equal to the square root of a quantity equaling the input impedance to said bridge times the real impedance of said electrodes under deposition conditions.
 5. The combination as defined in claim 2, further comprising, a housing for said bridge having separate enclosed compartments formed of electrically conductive walls and the inductor and capacitor of each said network being disposed in an individual one of said compartments for electrostatic and electromagnetic shielding therebetween.
 6. An RF sputtering system for particle deposition of a material on a workpiece, the combination comprising, a sputtering chamber having a pair of excitation electrodes for supporting material to be sputtered and adapted for containing a plasma of charged particles therein, a generator having an unbalanced output issuing an alternating voltage waveform, and an inductive-capacitive transformation bridge having an input connected to said generator output and a pair of outputs individually connected to said chamber electrodes issuing opposed phase components of said voltage waveform thereto and concurrently matching the impedance of said generator output with the dissipative impedance of said electrodes under the influence of said plasma.
 7. A system as defined in claim 6, said bridge comprising, a pair of inductors and a pair of capacitors alternately connected end to end and providing said bridge input across a first set electrically nonadjacent junctions and providing said outputs at the remaining electrically nonadjacent junctions relative to one of said first set junctions.
 8. A system as defined in claim 6, further comprising, an unbalanced coaxial transmission line connecting said generator output to said bridge input.
 9. A system as defined in claim 8, wherein said chamber is formed with electrically conductive wall and said bridge is mounted in an electrically conductive housing disposed in contiguous relation with said sputtering chamber and forming a reference ground therewith.
 10. In an RF sputtering method of depositing particles on a workpiece by exciting a source of material with an alternating electric field issued by electrodes mounted adjacent the source material and energized by an alternating voltage output of a generator to form a plasma of deposition particles the steps comprising, transforming the alternating voltage issued by said generator into opposing phase voltage components by applying said generator output to an inductive-capacitive bridge, applying said opposing phase voltage components separately to separate said electrodes by electrical connections to said bridge, and adjusting the impedance of said bridge to match the impedance of said generator output with the dissipative impedance of said electrodes while under the influence of said plasma.
 11. The steps as defined in claim 10, wherein said bridge is formed by alternate inductive and capacitive branches, and said steps of adjusting comprises, simultaneously varying the capacitances of said electrodes in the environment of said plasma. 